"A Celebration of JPL: the 20th Anniversary of the Voyager
Launches" will be the theme for a free public presentation to be
held at 7 p.m., Thursday, June 19, in JPL's von Karman
Auditorium, 4800 Oak Grove Drive, Pasadena. Seating is on a
first-come, first-served basis.

The twin Voyager 1 and Voyager 2 spacecraft were launched in
fall 1977 for a tour that would explore for the first time all of
the giant outer planets, their ring systems,and 48 of their
moons. Taking advantage of a rare geometric arrangement of the
outer planets in the late 1970s and into the 1980s, the Voyagers
were able to conduct a four-planet tour using a minimum of
propellant and trip time. This layout of Jupiter, Saturn, Uranus
and Neptune, which occurs about every 175 years, allowed the
Voyagers to swing from one planet to the next without the need
for large onboard propulsion systems.

During the 1979 encounter with Jupiter, Voyager 1 revealed a
faint, dusty ring around the giant planet, while the Great Red
Spot was imaged in detail as a complex storm moving in a
counterclockwise direction. The moon Io excited scientists and
the world with the first ever evidence of active volcanism on
another body in the solar system. Ganymede revealed cratered and
grooved terrain that suggested to scientists Ganymede's entire
icy crust had been under tension from global tectonic processes,
and Callisto showed the Voyagers an ancient, heavily cratered
crust showing faint rings left by enormous impact craters. Three
satellites, Adrastea, Metis and Thebe, were also discovered.

During the 1980 and 1981 encounters with Saturn, the
Voyagers imaged unexpected structures such as kinks and spokes in
Saturn's ring system. At Uranus in 1986, Voyager 2 found a
magnetic field tilted 60 degrees from the planet's axis of
rotation that twisted into a long corkscrew shape behind the
planet. Voyager 2 also found 10 new moons around Uranus, bringing
the total number to 15.

Long, bright clouds, similar to cirrus clouds on Earth, were
seen high in Neptune's atmosphere during Voyager 2's 1989
encounter. The strongest winds on any planet were measured on
Neptune. Triton, Neptune's largest moon, showed active geyser-
like eruptions spewing invisible nitrogen gas and dark dust
particles several kilometers into the planet's atmosphere.

Voyager 1 is now leaving the solar system, rising above the
ecliptic plane, while
Voyager 2 is heading out of the solar system below the ecliptic
plane. Now called the Voyager Interstellar Mission, the
spacecraft are returning data to characterize the outer solar
system environment and search for the heliopause boundary, the
outer limit of the Sun's magnetic field and outward flow of the
solar wind. Each Voyager spacecraft has seven science
instruments returning data, and flight controllers believe both
spacecraft will continue to operate and send back valuable data
until at least the second decade of the 21st century.

Presenting the evening talk will be JPL Director Dr. Edward
Stone, who is also the Voyager Project Scientist. He became the
Voyager Project Scientist in 1972, and has served as Director of
JPL since 1991. Stone has been a principal investigator on nine
NASA spacecraft missions and a co-investigator on five other NASA
missions, and has received two NASA Outstanding Leadership Medals
in addition to several more from both NASA and other
astronautical organizations. He is a fellow in the American
Astronautical Society, and holds both a master's degree and Ph.D
in physics from the University of Chicago.

This lecture is part of the von Karman Lecture Series held
monthly by the JPL Public Information Office. A web site on the
lecture series is located at http://www.jpl.nasa.gov/lecture. For
directions and other information, call the Public Information
Office at (818) 354-5011.